Xenon-135
135Xe is an unstable isotope of xenon with a half-life about 9.2 hours. 135Xe is a fission product of Uranium (yield 6.3%) which is the most powerful known neutron-absorbing nuclear poison (3 million barnes) and has a significant effect on nuclear reactor operation.
Xe-135 effects on restart
The inability of a reactor to be started due to the effects of Xe-135 is sometimes referred to as xenon precluded start-up. The period of time where the reactor is unable to override the effects of Xe-135 is called the xenon dead time. During periods of steady state operation, at a constant neutron flux level, the Xe-135 concentration builds up to its equilibrium value for that reactor power in about 40 to 50 hours. When the reactor power is increased, Xe-135 concentration initially decreases because the burn up is increased at the new higher power level. Because 95% of the Xe-135 production is from iodine-135 decay, which has a 6 to 7 hour half-life, the production of Xe-135 remains constant; at this point, the Xe-135 concentration reaches a minimum. The concentration then increases to the new equilibrium level for the new power level in roughly 40 to 50 hours. The magnitude, and the rate of change of concentration during the initial 4 to 6 hours following the power change, is dependent upon the initial power level and on the amount of change in power level; the Xe-135 concentration change is greater for a larger change in power level. When reactor power is decreased, the process is reversed[1]
135Iodine is a fission product of Uranium with a yield of about 1%. This 135I decays with a 6.7 hour half-life to 135Xe. Thus, in an operating nuclear reactor, 135Xe is being continuously produced. 135Xe has a very large neutron absorption cross-section, so in the high neutron flux environment of a nuclear reactor core, the 135Xe soon absorbs a neutron and becomes stable 136Xe. Thus, in about 50 hours, the 135Xe concentration reaches equilibrium where its creation by 135I decay is balanced with its destruction by neutron absorption.
When reactor power is decreased or shut down by inserting neutron absorbing control rods, the reactor neutron flux is reduced and the equilibrium shifts initially towards higher 135Xe concentration. The 135Xe concentration peaks about 11.1 hours after reactor power is decreased. Since 135Xe has a 9.2 hour half life, the 135Xe concentration gradually decays back to low levels over 72 hours.
The temporarily high level of 135Xe with its high neutron absorption cross-section makes it difficult to restart the reactor for several hours. The neutron absorbing 135Xe acts like a control rod reducing reactivity. The inability of a reactor to be started due to the effects of Xe-135 is sometimes referred to as xenon precluded start-up. The period of time where the reactor is unable to override the effects of Xe-135 is called the xenon dead time.
If sufficient reactivity control authority is available the reactor can be restarted but a Xenon burn-out transient must be carefully managed. As the control rods are extracted and criticality is reached, neutron flux increases many orders of magnitude and the 135Xe begins to absorb neutrons and be transmuted to 136Xe . The reactor burns off the nuclear poison. As this happens, the reactivity increases and the control rods must be gradually re-inserted or reactor power will increase. The time-constant for this burn-off transient depends on the reactor design, power level history of the reactor for the past several days, and the new power setting. For a typical step up from 50% power to 100% power, 135Xe concentration falls for about 3 hours [2]. Failing to manage this Xenon transient properly caused the Chernobyl reactor power to overshoot ~100x normal causing a steam explosion.[3] The Xenon burn-out rate is proportional to neutron flux and thus reactor power. If reactor power doubles, the Xenon burns out twice as quickly. The larger the rate of increase in reactor power, the faster the Xenon burns out and the more quickly reactor power increases.
Reactors using continuous reprocessing like many molten salt reactor designs might be able to extract Xe-135 from the fuel and avoid these effects[citation needed].
Decay and capture products
135Xe that does not capture a neutron decays to Cs-135, one of the 7 long-lived fission products, while 135Xe that does capture a neutron becomes stable 136Xe. Estimates of the proportion of Xe-135 during steady-state reactor operation that captures a neutron include 90%[4], 39%-91%[5] and "essentially all"[6].
136Xe from neutron capture ends up as part of the eventual stable fission xenon which also includes 136Xe, 134Xe, 132Xe, and 131Xe produced by fission and beta decay rather than neutron capture.
133Xe, 137Xe, and 135Xe that has not captured a neutron all beta decay to isotopes of caesium. Fission produces 133Xe, 137Xe, and 135Xe in roughly equal amounts, but after neutron capture, fission caesium will contain more stable 133Cs (which however can become 134Cs on further neutron activation) and highly radioactive 137Cs than 135Cs.
See also
References
- ^ DOE Fundamentals Handbook: Nuclear Physics and Reactor Theory, pages 35-42.
- ^ Xenon decay transient graph
- ^ Chernobyl_disaster
- ^ CANDU Fundamentals: 20 Xenon: A Fission Product Poison
- ^ http://www.risoe.dk/rispubl/reports_INIS/RISOM2437.pdf Utilization of the Isotopic Composition of Xe and Kr in Fission Gas Release Research
- ^ http://www.c-n-t-a.com/srs50_files/049roggenkamp.pdf The Influence of Xenon-135 on Reactor Operation